Epigenetic silencing of the intronic microRNA hsa-miR-342 and its host gene EVL in colorectal cancer

MicroRNAs are small, non-coding RNAs that influence gene regulatory networks by post-transcriptional regulation of specific messenger RNA targets. MicroRNA expression is dysregulated in human malignancies, frequently leading to loss of expression of certain microRNAs. We report that expression of hsa-miR-342, a microRNA encoded in an intron of the gene EVL, is commonly suppressed in human colorectal cancer. The expression of hsa-miR- 342 is coordinated with that of EVL and our results indicate that the mechanism of silencing is CpG island methylation upstream of EVL. We found methylation at the EVL/hsa-miR-342 locus in 86% of colorectal adenocarcinomas and in 67% of adenomas, indicating that it is an early event in colorectal carcinogenesis. In addition, we observed a higher frequency of methylation ( 56%) in histologically normal colorectal mucosa from individuals with concurrent cancer compared to mucosa from individuals without colorectal cancer ( 12%), suggesting the existence of a 'field defect' involving methylated EVL/hsa-miR-342. Furthermore, reconstitution of hsa-miR- 342 in the colorectal cancer cell line HT-29 induced apoptosis, suggesting that this microRNA could function as a proapoptotic tumor suppressor. In aggregate, these results support a novel mechanism for silencing intronic microRNAs in cancer by epigenetic alterations of cognate host genes.